29 - Internal and external conical refraction

Summary

The phenomenon of conical refraction was predicted by Sir William Rowan Hamilton in 1832 and its existence was confirmed experimentally two months later by Humphrey Lloyd. (James Clerk Maxwell was only a toddler at the time.) The success of this experiment contributed greatly to the general acceptance of Fresnel's wave theory of light.

Conical refraction has been known for nearly 170 years now, and a complete explanation based on Maxwell's electromagnetic theory has emerged, which is accessible through the published literature. The complexity of the physics involved, however, is such that it prevents us from attempting to give a simple explanation. We shall, therefore, confine our efforts to presenting a descriptive picture of internal and external conical refraction by way of computer simulations based on Maxwell's equations.

Overview

To observe internal conical refraction one must obtain a slab of biaxial birefringent crystal, such as aragonite, that has been cut with one of its optic axes perpendicular to the polished parallel surfaces of the slab (see Figure 29.1). When a collimated beam of light (say, from a HeNe laser) is directed at normal incidence towards the front facet of the slab, the beam enters the crystal and spreads out in the form of a hollow cone of light. Upon reaching the opposite facet, the beam emerges as two concentric hollow cylinders, propagating in the same direction as the original, incident beam.